<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <style>
    body,html {
      width: 100%;
      height: 100%;
    }
    #a1 {
      width: 50%;
      height: 50%;
      background-color: blue;
    }
    #a2{
      width: 400px;
      height: 340px;
      background-color: red;
    }
  </style>
</head>
<body>
      <div id="a1"></div>
      <div id="a2"></div>
</body>
</html>